title: Structure-property relationships and (1)O)2) photosensitisation in sterically encumbered diimine Pt(II) acetylide complexes.
abstract: A series of sterically encumbered [Pt(L)(σ-acetylide)2 ] complexes were prepared in which L, a dendritic polyaromatic diimine ligand, was held constant (L=1-(2,2'-bipyrid-6-yl)-2,3,4,5-tetrakis(4-tert-butylphenyl)benzene) and the cis ethynyl co-ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron-withdrawing phenyl-CF3 substituents (4) with electron-donating pyrenes (5) resulted in a red shift of both the lowest-energy absorption (ΔE=3300 cm(-1) , 61 nm) and emission bands (ΔE=1930 cm(-1) , 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited-state lifetimes, switched from being (3) MMLL'CT-derived (mixed metal-ligand-to-ligand charge transfer) when phenyl/polyphenylene substituents (3, 4, 6) were present, to ligand-centred (3) ππ* when the substituents were more conjugated aromatic platforms [pyrene (5) or hexa-peri-hexabenzocoronene (7)]. The novel Pt(II) acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited-state lifetimes suggested they would be good candidates for use as singlet-oxygen photosensitisers. Determined by in situ photooxidation of 1,5-dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl-5 as sensitiser (kobs =39.3×10(-3)  min(-1) ) was over half that of the known (1) O2 sensitiser tetraphenylporphyrin (kobs =78.6×10(-3)  min(-1) ) under the same conditions. Measured (1) O2 quantum yields of complexes 5 and 7 were half and one-third, respectively, of that of TPP, and thus reveal an efficient triplet-triplet energy-transfer process in both cases.
authors.affiliation: School of Chemistry, University of Dublin, Trinity College, College Green, Dublin 2 (Ireland), Fax: (+353) 1-6712826.
authors.forename: Deanne
authors.initials: D
authors.lastname: Nolan
authors.forename: Belén
authors.initials: B
authors.lastname: Gil
authors.forename: Longsheng
authors.initials: L
authors.lastname: Wang
authors.forename: Jianzhang
authors.initials: J
authors.lastname: Zhao
authors.forename: Sylvia M
authors.initials: SM
authors.lastname: Draper
keywords: acetylide ligands; chelates; photochemistry; photophysics; platinum